‘Lotus Effect’ Studied in Real Time

As a result of its novel ability to resist getting dirty, the lotus is symbolic worldwide of spiritual purity. The microstructured surface of hairy bumps on lotus leaves causes water droplets that fall on them to bead into large drops that roll away, taking with them any particulates that are present.

Automotive engineers are interested in this self-cleaning property, known as the “lotus effect,” for various applications, including the development of materials for windshields and body panels that never require washing. But such materials would present a dilemma. After all, how can one tint a windshield or paint a car body that sheds applied substances?

Electron microscopy of water condensation and evaporation on a lotus leaf offers insight into the plant’s self-cleaning properties, which have potential applications in the design of materials for the automotive industry and other fields. The leaves feature microscale bumps covered with nanoscale hairs (top). The structures increase the contact angle of coalescing water droplets (middle), causing them to roll off the leaf. Under some circumstances, however, the drops can adhere to the leaf and evaporate (bottom). Courtesy of Yang-Tse Cheng, General Motors Research and Development Center. Images by Daniel E. Rodak, Ricardo Meda Technical Services LLC.A potential solution could be the result of another property of the lotus: Although drops that fall on the plant never seem to get it wet, drops that condense on it can be quite sticky. Researchers at General Motors Corp.’s Research and Development Center in Warren, Mich., at Ricardo Meda Technical Services LLC in Southfield, Mich., and at GM’s Fuel Cell Activities unit in Honeoye Falls, N.Y., thus applied real-time electron microscopy to the study of the condensation of water on lotus leaves. After discovering that the drops can stick to a leaf under some circumstances, they developed a geometric model to describe the phenomenon.

Using a Quanta 400 scanning electron microscope from FEI Co. of Hillsboro, Ore., in environmental mode, the investigators observed water that condensed on leaves cooled to 5 °C. Although the droplets coalesced into bigger drops and were lifted onto the hairy bumps so that they had large apparent contact angles, they would not roll or slide off the leaves even when tilted by 45°.

By reducing the pressure in the sample chamber, the researchers caused the drops to evaporate. In the process, large drops shrank until they could no longer be supported by the bumps and fell onto the flat surface between the structures, adhering there until they evaporated.

The scientists propose that, although the drops produced through condensation have large apparent contact angles, their actual local contact angles with respect to the sides of the bumps are small. This is the case, they suggest, when the size of a drop is comparable to that of the roughness of the surface.